I think Mesa Gearden has been brought up in the Cactus forum a couple of times. They have seeds and plants.

Here is a link that might be useful: Mesa garden

You might try High Country Gardens. highcountrygardens.com

I've always had great luck with Highland Succulents. They have quality plants and good variety. They will be back online and shipping as weather allows beginning in early March.

Here is a link that might be useful: Their site

Arid Lands is one of my favorites.
Mike

Here is a link that might be useful: Arid Lands